1. Field of the Invention
This invention relates to a method for forming crystalline thin film having good crystallinity to be used for semiconductor devices such as image inputting device, image displaying device, photoelectric converting device, photographing device, etc.
2. Related Background Art
In recent years, it has been the practice to form a semiconductor device by depositing a crystalline semiconductor thin film on an amorphous substrate with a large area such as so called glass, and forming a desired pattern thereon which has been also provided for practical use. For example, a polycrystalline silicon film has been used as the driving device for liquid crystal display, etc., and for formation thereof, primarily the normal pressure CVD method, the low pressure CVD (LPCVD) method, the plasma CVD method, etc. have been employed, generally the LPCVD method has been employed widely and industrialized.
It is effective to reduce the amount of the grain boundaries existing in the semiconductor layer of the semiconductor device, namely essentially required to increase the grain sizes in the polycrystalline film.
However, in the LPCVD method as mentioned above, when an amorphous substrate is used, the grain size can be increased only to about the height of 0.1 .mu.m-0.3 .mu.m even if the substrate temperature may be made about 1000.degree. C. (T. I. Kammins & T. R. Cass, Thin solid Film, 16 (1973) 147).
In contrast, enlargement of grain size in polycrystalline silicon film has been attempted variously in recent years. For example, there have been investigated the method in which, after deposition of a thin film by an energy beam such as laser, electron beam, lamp, band-shaped heater, etc., the thin film is subjected to heat treatment in the solid phase as such or melting recrystallization (Single crystal silicon on non-single-crystal Insulators. Journal of crystal growth, vol. 63, No. 3, October, 1983, elected by G. W. Cullen) and the method in which after deposition of a thin film, the thin film is once made amorphous by ion implantation and then subjected to solid phase growth (T. Nagai, H. Hayasi, T. Ohshima, Journal of Electro-chemical Society, 134 (1987) 1771) . According to these methods, a polycrystalline silicon thin film having larger grain sizes as compared with the LPCVD method as described above has been obtained, and particularly in the latter method, one having a grain size of about 5 .mu.m is obtained. Further, among electric field effect transistors prepared by the polycrystalline silicon thin films formed by these methods, one having an electron mobility to the same extent as in the case when prepared by use of a single crystalline silicon has been also reported.
However, many problems are involved in complicatedness, controllability, speciality, ease of the steps in the grain size enlargement techniques as described above. Also, since the process temperature becomes high, there is also the problem that no inexpensive glass substrate can be employed, and therefore they are unsuitable for forming easily a polycrystalline silicon thin film with large grain size and little grain boundary at a large area.
On the other hand, according to the plasma CVD method, it is possible to lower the substrate temperature (&lt;400.degree. C.) and also enlargement of area can be easily done, but the silicon film formed has a grain size of 0.05 .mu.m or less, and also has a structure containing amorphous phase in the film, even when formed into amorphous or polycrystalline film, whereby the carrier mobility of electrons becomes 1 cm.sup.2 /v sec to accomplish no desired high performance semiconductor device (W. E. Spear, G. Willeke, P. G. LeComber, A. G. Fitzgerald, Journal de Physique, C4 (1981) 257).
For solving such problems of the plasma CVD method, the method of forming a polycrystalline silicon film by use of the HR-CVD method (Hydrogen Radical Assisted CVD Method) disclosed in Japanese Patent Laid-open Application No. 62-241326 has been proposed, but the crystal grain size in the film immediately after formation of deposition film is about 0.2 .mu.m, which is slightly improved as compared with the plasma CVD method, and it is necessary to impart heat or light energy during or after formation of the film in order to enlarge further the grain size (Japanese Patent Laid-open Application Nos. 62-240766, 62-240767, 62-243767).
In the films as described above, the grain size enlargement technique of polycrystalline silicon film of the prior art depends greatly on many steps and high temperature process, and for supplying semiconductor devices demanded to have high performances such as image display device, photoelectric converting device, etc. widely to the market, it has been desired to have a method for forming a polycrystalline silicon film with good crystallinity and large grain size, namely with little defect, on a large area non-single-crystal substrate at low temperature easily and stably.